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Flavor/fragrance qualities of commercial food products are greatly dependent on the volatile and semi-volatile organic compounds present both in the matrix and the headspace aroma. Today, there is a problem in the food industry as to what constitutes the optimum conditions for extracting and handling honey so as to provide a consistent quality, flavor and aroma of honey to the consumer. Many factors must be considered such as differences between bee colonies, how they are managed by the beekeepers for a specific honey and time of harvest. The most important factor are the plants used by the bees for the production of honey. The flavor and color of the honey depends on the type of plant that produces a nectarine honey. For example, sourwood produces a honey that is different in color and flavor from that of clover or poplar. In addition, verification of the honey as to its plant source is a primary concern. Analytical techniques are needed to profile and identify flavors, fragrances, off-flavors, off-odors and potential contaminants that may be present as flavor and fragrance additives. Volatile organic compounds were collected from samples of natural honey using a purge and trap technique (P&T), followed by trapping on an adsorbent resin and subsequent analysis by thermal desorption-gas chromatography- mass spectrometry (TD-GC-MS).
All experiments were conducted using a Scientific Instrument Services model TD-2 Short Path Thermal Desorption System accessory connected to the injection port of an HP 5890 Series II GC interfaced to an HP 5971 Mass Selective Detector (1,2). The mass spectrometer was operated in the electron impact mode (EI) at 70eV and scanned from 35 to 400 daltons during the GC run for the total ion chromatogram.
A short 0.5 meter by 0.53 mm diameter fused silica precolumn was attached to the injection port end of a 30 meter x 0.25 mm i.d. DB-5MS capillary column containing a 0.25µm film thickness. The GC injection port was set to 300 degrees C and a 10:1 split was used during the injection step. The GC oven was maintained at -40 degrees C during the desorption and extraction process, after which the oven was temperature programmed from -40 degrees C to 300 degrees C at a rate of 10 degrees per minute for the total ion chromatogram.
Figure 1 - Purge & Trap Apparatus
Four honeys (thistle, tulip poplar, sourwood & mountain laurel) harvested from different areas of Virginia and a tupelo honey harvested in the Southeastern United States were analyzed to compare the flavor profiles and to quantify the volatile organics that were present. The thistle and tupelo honeys were several years old (at least 2+), the poplar was approximately a year old and the mountain laurel approximately 10 months old. The sourwood and mountain laurel honeys contained some wax flakes as they were packed as chunk honey (with pieces of filled comb). For quantification, an internal standard was spiked into the adsorbent traps after the sample had been isolated. No correction for extraction efficiency of recovery is achieved using this technique; however, it functions as a useful means of quantifying the levels of components present on the adsorbent traps (3).
Sample sizes of 5-7 g of natural honey were pipetted into a 10 ml test tube and heated to 60 degrees C. Samples were sparged with high purity helium at 20 ml/min with an additional 20 ml/min dry purge for 45 minutes using a Scientific Instrument Services Purge and Trap System (Fig. 1). Volatile analytes were gas extracted and carried to a preconditioned 4.0 mm i.d. glass-lined stainless steel desorption tube packed with 100 mg of Tenax® TA. Once the samples were collected, they were spiked with 100 ng of d-14 cymene internal standard by injecting 1ml of a 100 ng/ml of a d-14 cymene stock solution in methanol by syringe injection into the Tenax matrix.
The desorption tubes with sample and internal standard were then attached to the Short Path Thermal Desorption System and a syringe needle attached. The desorption tube was injected into the GC injection port at desorption block temperatures of 220 degrees C for 10 minutes at a purge flow of 10 ml/min, and a GC injection split ratio of 10:1.
Results and Discussion
Five honeys produced from different plants were analyzed to identify, compare and quantify the volatile organics present. Typical honeys exhibited from 50 to more than 100 peaks in a chromatogram, which were identified via the mass spectrometer. The honeys were found to contain numerous mono- and sesquiterpenoid compounds and flavors such as benzaldehyde, furfural, isovaleraldehyde and phenylacetaldehyde (Figs. 2-6). The flavor compounds benzaldehyde and furfural have an almond-like odor, isovaleraldehyde has an apple-like odor, and phenylacetaldehyde possesses an odor reminiscent of lilac and hyacinth. In addition, the presence of the branched aldehydes 3-methyl-butyraldehyde and 2-methyl-butyraldehyde in each of the honeys reflected the microbial quality of the honey.
Figure 2 - Tupelo Honey (5.24g). Purged For 45 min at 20 ml/min Dry Purge On Tenax TA Adsorbent Trap Followed By Thermal Desorption At 220 Degrees C For 10 min.
Trace amounts of the compound lilac aldehyde were detected in each of the honeys. Lilac aldehydes were first isolated from the lilac flower oil (4) and later identified as fragrant components in gardenia flower (5), in Platanthera strict (6), and in Artemisia pallens (7). Lilac alcohols have also been isolated from lilac flower oil (4) and determined to be fragrant components of gardenia flowers (5) and A. pallens (7). Linalool oxide, an isomer of lilac alcohol, and tetrahydro-furfuryl-(2)-alcohol were detected in each of the honeys (Figs. 2-6) with much higher concentrations found in the two year old plus honeys, tupelo and thistle (Figs. 2&3), while only trace amounts of these compounds were detected in the 10 month old sourwood and mountain laurel honeys (Figs. 5&6). Conversely, high concentrations of the compounds 3,5,5-trimethyl-2-cyclohexen-1-one and 2-cyclohexen-1-one were found in the 10 month old sourwood and mountain laurel honeys with only trace amounts present in the several year old tupelo and thistle honeys. The one year old tulip poplar honey contained each of the compounds linalool oxide, tetrahydro-furfuryl-(2)-alcohol, 3,5,5-trimethyl-2-cyclohexen-1-one and 2-cyclohexen-1-one (Fig. 4) and appeared to be an intermediate between the 10 month old and two year old plus honeys with respect to age of the honey. A case may be made for separating the honeys according to their age in this study; however, many other factors must be considered such as plant species, differences between bee colonies, how they are managed and time of harvest. The 10 month old sourwood and mountain laurel honeys also contained some wax flakes as they were packed as chunk honey with pieces of filled comb.
Figure 3 - Thistle Honey (6.41g). Purged For 45 min At 20 ml/min Dry Purge On Tenax TA Adsorbent Trap Followed By Thermal Desorption At 220 Degrees C For 10 min.
Trace amounts of octane and the aliphatic C6 compound hexanal were detected in each of the honeys. It is generally believed that octane concentrations increase with time during storage and the formation of C6 aldehydes and alcohols in the plant is related to cell destruction. It has been assumed for a long time that unsaturated fatty acids are the precursors of C6 aliphatic compounds. In addition, trace amounts of the aliphatic compounds octanal, nonanal and decanal were identified in each of the honeys. It has been suggested that several aldehydes and ketones formed by the oxidation of fatty acids, especially linoleic and linolenic acids, may be of importance for the development of rancid flavour (8)
Figure 4 - Tulip Poplar honey (5.0g). Purged For 45 min at 20 ml/min Dry Purge On Tenax TA Adsorbent Trap Followed By Thermal Desorption At 220 Degrees C For 10 min.
Figure 5 - Sourwood honey (5.85g). Purged For 45 min at 20 ml/min Dry Purge On Tenax TA Adsorbent Trap Followed By Thermal Desorption At 220 Degrees C For 10 min.
Diacetyl, found in bay and other oils, was detected in mountain laurel and is used as a carrier of aroma (Figs. 6). The aliphatic compound isoamyl alcohol was found in the tulip poplar honey (Fig. 4). In addition, the aromatic compound toluene was detected in each of the honeys. (Figs. 2-6). The presence of this compound can occur naturally from essential essences. High concentrations of the flavors cis-jasmone and beta-damascenone were found in tupelo honey (Fig. 2). Cis-jasmone present in the volatile portion of oil from Jasmone flowers has an odor of jasmine while beta-damascenone has a nutmeg-like odor. Other middle-chain aliphatic alcohols, aldehydes, ketones and furan derivatives which reflect the microbiological purity and storage conditions of the honeys, were also found in each of the honeys.
Figure 6 - Mountain Laurel Honey (5.57g) Purged For 45 min At 20 ml/min Dry Purge On Tenax TA Adsorbent Trap Followed By Thermal Desorption At 220 Degrees C For 10 min.
The Short Path Thermal Desorption System used in conjunction with the Purge and Trap System permits the identification and quantification of trace levels of volatile organics in honeys. The method described above permits both the qualitative and quantitative analysis of natural honeys to identify the volatile organics, flavoring compounds, contaminates, and off-odor compounds present. This technique has proven effective in detecting and identifying a larger number of organic compounds at concentrations lower than were previously obtainable via other analysis techniques, as static headspace GC analysis. It also represents a tremendous improvement over the time-consuming solvent extraction techniques normally used in the laboratory. This technique can easily be incorporated into a troubleshooting technique to detect problems in a wide variety of commercial food products, to compare various competing manufacturers products, as well as a quality control program. This technique has also been applied to other applications such as quantification of benzene and toluene in food products (3), and flavors and fragrances in food products (9,10), commercial products (6) and plant material (11).
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Yttria coated filament at start
Yttria coated filament after 16,000 cycles